Parsing logic Expression with Javascript

Question

I'm building a logic expression parser. I really like the way mongoDB structures their logic statements. However, I'm thinking if this is the best structure to store and parse logic expressions.

const logicPayload = [
  { varA: { $eq: "meow" } },
  { $and: [{ varB: { $lt: "varD" } }, { varB: { $gte: "varC" } }] },
  { $not: [{ varB: { $eq: "varA" } }] }
];

Given the payload above, how can i design a function to parse the expression. varA, varB varC and varD will be resolve at runtime.

A dummy resolve function is below.

const resovleValue = varID => {
  switch (varID) {
    case "varA":
      return "meow";
      break;
    case "varB":
      return 100;
    case "varC":
      return 100;
    case "varD":
      return 983;
  }
};

I'm trying to create the logic function below

/**
 * This logic function must only return true or false
 */
const logicFunction = logicPayload => {};

Any guidance is much appreciated. thanks.

LukStorms · Accepted Answer · 2020-02-03 11:28:24Z

2

Here's a logic parser that uses recursion to deal with the internal arrays of logic.

It'll stop processing as soon it finds a rule that returns false.

function checkRule(rule) {
  let key1 = Object.keys(rule)[0];
  let comp = Object.values(rule)[0];
  
  if(/^[$](and|or|not)$/.test(key1) && Array.isArray(comp)) {
    let aon = key1;
    let rules = comp;
    let results = [];
    
    for(r of rules){
      let valid = checkRule(r);
      results.push(valid);
      //console.log(`${aon} ${JSON.stringify(r)}: ${valid}`);
      if(aon === '$and' && !valid) return false;
      if(aon === '$or' && valid) return true;
      if(aon === '$not' && valid) return false;
    }
     //console.log (JSON.stringify(results));
     if(aon === '$and' && results.some((x)=>!x))
         return false;
     if(aon === '$or' && results.every((x)=>!x))
         return false;
     if(aon === '$not' && results.some((x)=>x))
         return false;
}
else {
  let operator = Object.keys(comp)[0];
  let key2 = comp[operator];
  let val1 = resolveValue(key1) || key1;
  let val2 = resolveValue(key2) || key2;
  //console.log(`\t${val1} ${operator} ${val2}`);
  
  switch(operator) {
    case '$eq': 
     if(!(val1 == val2)) return false;
     break;
    case '$ne': 
    case '$neq':
     if(!(val1 != val2)) return false;
     break;
    case '$lt': 
     if(!(val1 < val2)) return false;
     break;
    case '$le': 
    case '$lte': 
     if(!(val1 <= val2)) return false;
     break;
    case '$gt': 
     if(!(val1 > val2)) return false;
     break;
    case '$ge':
    case '$gte': 
     if(!(val1 >= val2)) return false;
     break;
    default: 
      return false;
   }
    
  }
  
  return true;
}

function logicFunction (logicPayload) {
   let results = [];
   
   for (rule of logicPayload) {
   	 let valid = checkRule(rule);
   	 //console.log(`${valid}: ${JSON.stringify(rule)}`);
   	 results.push(valid);
   	 if(!valid) return false;
   }
   return results.every((x)=>x);
};

const resolveValue = (varID) => {
  switch (varID) {
    case "varA": return "meow";
    case "varB": return 100;
    case "varC": return 100;
    case "varD": return 983;
  }
};


let logicPayload = [
  { varA: { $eq: "meow" } },
  { $and: [
      { varB: { $lt: "varD" } },
      { varB: { $gte: "varC" } }
   ] },
  { $not: [ { varB: { $eq: "varA" } } ] }
];

 let result = logicFunction(logicPayload);
 console.log(result);

edited Feb 3, 2020 at 11:28

answered Feb 2, 2020 at 19:14

LukStorms

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6 Comments

mingsterism Over a year ago

thanks for the fast response and for that comprehensive answer. would like to ask, any opinion on using the current payload vs changing it to this

const logicPayload = [  [varA, "$eq", "meow"],  [   "$and",    [      [varB, "$lt", "varD"],      varB, "$gte", "varC"]    ]  ],  ["$or", [varB, "$notEq", varA], [varC, "$gt", varD]]

Am wondering which is the better structure and why. thanks for the guidance

LukStorms Over a year ago

Well, in the current version it checks if it's array to see if it's an $and/$not/$or. So you'd have to use an other trick. Should be possible, f.e. checking if there's a $ in the first element. Btw, I noticed that in the logicPayload it's hard to know if it's a string or a variable. F.e. what if you have a "meow" variable.

LukStorms Over a year ago

Actually, in ES6 compatible code you could use Class for the equations. But I'm guessing you're looking more for a format that can be send as a JSON string.

mingsterism Over a year ago

since both methods are possible, which you believe is easier, better. how can I access if arrays or objects work better. in terms of the variables (keys) in logicPayload, i'm checking if it's a mongoID string. so all variables need to pass ObjectId.isValid( variable) so that's why we have another function for resolving the variable by querying the mongo database.

LukStorms Over a year ago

Personal I find the first, with objects in arrays, more readable.

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